This blog is updated daily..
A general description is here.
Indexing with a vector index to [[]] has now been extended to all recursive types.
pairlists may now be assigned as elements of lists. (Lists could always be created with pairlist elements, but [[<- didn't support assigning them.)
The perl=TRUE versions of strsplit(), grep() and friends now work in a non-UTF-8 MBCS locale, by translating the inputs to UTF-8.
C99 support for MBCS is now required, and configure option --disable-mbcs has been withdrawn.
A suitable iconv() is now required: support for configure option --without-iconv has been withdrawn (it was deprecated in R 2.5.0).